Electricity generation from solar energy suffers from a fundamental problem of reliance on an unpredictable and variable source of energy. To some extent this problem can be mitigated by using thermal energy storage systems which can extend the availability of solar energy beyond daylight hours, and even to 24 hours if the storage is sufficiently large, but after periods of low or no radiation even the storage system will not be able to provide the energy required to render the plant as capable of delivering base-load electricity reliably. Put simply, it is not feasible realistically to build an electric generation capacity using solar energy that is fully base-load capable and dispatch manageable without supplementing the solar energy with some other source of thermal energy.
The approach taken then is to supplement the thermal energy developed from solar radiation with thermal energy from an auxiliary source. This source is usually a fossil fuel such as liquefied natural gas (LNG), liquefied petroleum gas (LPG) or distillates. These fuels are favoured because the boilers and heaters which burn these fuels can be designed for quick start and stop, cycling duties and low load duties. These characteristics complement the variability and unpredictability of solar radiation, and yield a stable generation capacity.
However, this type of system does not address the fundamental concern of establishing a truly renewable source of base-load electricity generation, reliant as it is on the non-renewable fossil fuel for levelling and supplementing production.
The alternative is to use a renewable auxiliary fuel, such as biomass. This presents a problem of configuration and control, which is addressed by the present invention.
Effective hybridization of a thermal solar field with a biomass system involves melding two distinct sources of thermal energy, and delivering that energy seamlessly to the power block for the generation of electricity. A general scheme of such a process is shown schematically in FIG. 1.
Thus, in FIG. 1, thermal energy is obtained from a solar field 10 that uses solar radiation as the prime energy source (CSP) and from a biomass system 11 that uses energy source such as a wood fire, organic waste municipal waste and other non-solar thermal energy sources (BM). The thermal energy from these two distinct sources is fed to a hybridization system 15 that facilitates the combination of two systems that feature different characteristics, namely that solar energy fluctuates in accordance with the sun insolation, while the biomass system is limited in its ability to follow the solar system variations. The hybridization system 15 feeds thermal energy which can be eventually used by a turbo-generator 16 that produces electricity 17 or by other thermal energy users.
FIG. 2 shows schematically a biomass fired boiler 20 arranged in parallel with a solar steam generator 21 and can be used to supplement steam output either partially or completely in times of little or no solar radiation prior to feeding to a turbine (such as 16 in FIG. 1). Such an arrangement combines in a simple fashion the two technologies with system integration at the point of steam supply to the turbine. Heat transfer fluid 22 from the solar field 10 is fed via a first valve 23 to the solar steam generator 21 and cooler heat transfer fluid 24 is pumped back to the solar field by a pump 25. Steam 26 produced by the solar steam generator 21 is fed in parallel with steam produced by the biomass fired boiler 20 to the turbine and condensate 27 is returned to both the biomass fired boiler 20 and the solar steam generator 21 where it is re-heated in an ongoing cycle. In order to exploit solar thermal energy even in the absence of solar insolation, thermal energy storage (TES) 28 may be coupled to the solar steam generator 21 via a second valve 29. Solar thermal energy stored in the TES 28 during daylight hours may be used at night to power the solar steam generator 21 by closing the first valve 23 and opening the second valve 29.
However, the efficacy of this solution demands from the boiler a partial-load (turndown) capability as well as a “rapid response” startup capability in response to supplementary thermal demand by the turbine when the solar field cannot deliver 100% of steam requirements. Typically, the combustion chamber of a biomass boiler can only be heated up at a limited rate (around 100° C. per hour), meaning the startup time from cold is several hours. Each startup entails thermal losses until the system is up to operating conditions. In applications experiencing frequent starts and stops—such as the case here—the startup losses mount significantly.
In addition, the biomass boiler turndown is usually limited to 50%-70% of full load. Thus, during base-load operation when the boiler may be required to rapidly deliver steam to supplement the solar field output, the boiler must be kept “hot”, but running it at a minimal load is not feasible. Systems employing such an approach need to be kept hot using some sort of external heat source. The resultant energy losses are significant, reducing overall plant conversion efficiency.
Moreover, in a dispatch scenario where the plant is required to deliver power only during certain hours of the day (e.g. 7:00 to 24:00) this boiler solution would require frequent starts and stops, which would aggravate the inherent problems of biomass boilers noted above and also lower its utilization factor.
Thus while simple, this option does not offer a technically suitable configuration for an efficient base load plant.
FIG. 3 shows schematically a biomass fired steam super-heater 30 arranged in series with the solar steam generator 21. In other respects the arrangement is the same as shown in FIG. 2 and is therefore not described. This configuration aims to overcome one of the efficiency limitations of concentrated solar power based plants, namely the maximum steam temperature that can be achieved. Saturated steam from the solar field can be superheated to 540° C., for example, and thus improve thermal efficiency of the turbine from approximately 39% to 44%.
In order to do so, the super-heater 30 would need to track the steam output from the solar steam generator and deliver a proportional amount of energy for superheating. As noted, modulation of biomass boilers is problematic and so this cannot be done effectively. Moreover, although this arrangement increases cycle efficiency, it does not assist in stabilizing base-load generation or meeting the changing demand requirements. It would therefore be desirable to provide improved hybridization of the two energy sources, so as to improve their utilization efficiency as well as the utilization of the costly equipment comprising the hybrid power plant.
US 2009/205335 discloses a domestic energy supply system wherein the thermal energy of the temperature difference between at least one heat source and at least one heat sink is converted into work by way of a hybrid thermal engine. The thermal engine has a fluid cycle with at least two reservoirs, which, in each case as a condenser to be cooled or an evaporator to be heated, are thermally coupled to the heat source or the heat sink. U.S. Pat. No. 5,444,972 discloses a power plant which uses hydrocarbon fuels in conjunction with solar power to produce electricity.